1. Field of the Invention
This invention relates to an image reading apparatus having light condensing means such as a Fresnel lens for causing an image information light to enter imaging means for imaging the light on a light-receiving portion. In particular, the invention relates to an apparatus for reading an image by slit exposure or an image reading apparatus using a convergent type transmitting member array as imaging means.
2. Related Background Art
In recent years, the projection of an image light onto a Fresnel lens or the like to read the image of a copying apparatus, an image reader or the like has been contemplated as shown in Japanese Laid-Open Utility Model Application No. 162139/1983, Japanese Laid-Open Utility Model Application No. 27550/1984, Japanese Laid-Open Patent Application No. 198439/1984, Japanese Laid-Open Patent Application No. 198440/1984, U.S. patent application Ser. No. 602,012 (filed on Apr. 19, 1984), U.S. Pat. No. 4,120,580 and U.S. Pat. No. 4,027,962.
In such an apparatus, it is the usual practice to cause a projected light beam to travel toward an imaging lens for forming an image on the light-receiving portion by a condensing lens such as a Fresnel lens.
As such light condensing means, use has been made of a lens whose refractive power is rotation-symmetrical. However, in an apparatus wherein an image light is obtained by slit exposure for the purpose of making the apparatus compact, particularly an apparatus in which a convergent light transmitting member array comprising small lenses (particularly of short focus) arranged in the form of an array is used as imaging means, the quantity of received light has been insufficient or the received light image has been disturbed in some cases.
The cause of the problem occurring when use is made of a convergent light transmitting member array will hereinafter be described with reference to FIGS. 5A and 5B of the accompanying drawings.
FIG. 5A is a schematic cross-sectional view taken in the lengthwise direction of the array.
In the figure, reference numeral 1 designates an original having image information. The original 1 is illuminated by an illuminating light source, not shown. Reference numeral 5 denotes a projection lens for projecting an image information light. The projected light from this projection lens 5 is condensed by a Fresnel lens 7 and directed to a convergent light transmitting member array 9. This array 9 causes the image information light to be imaged on a photosensitive medium 10 which is a light-receiving portion. Reference numeral 8 designates an original supporting table for supporting an original thereon. The original supporting table 8 is capable of reading the image of an original 1 as well as the image of the original supported on the original supporting table 8.
Where a convergent light transmitting member array is used, to effectively direct a light beam to the photosensitive medium 10, it is preferable that as shown in FIG. 5A, in the lengthwise direction of the array 9, the light beam from the projection lens 5 be caused to travel in the direction of the optical axis of the individual convergent type light transmitting members, i.e., in a direction substantially orthogonal to the original supporting table 8.
Also, in a direction perpendicular to the lengthwise direction of the array 9, the light beam having reached the original supporting table 8 with the aid of the projection lens 5 may preferably be caused to travel toward the pupil of the convergent light transmitting member array 9. However, the refractive powers of the Fresnel lens 7 in both directions are equal and therefore, when the refractive power is made proper in the lengthwise direction of the array 9 (the state shown in FIG. 5A), there occurs a phenomenon that in a direction orthogonal to the lengthwise direction of the array 9, the light does not effectively enter the convergent light transmitting member array as shown in the cross-sectional view of FIG. 5B.
Therefore, loss of the quantity of light occurs and the problem of insufficient light occurs in the light-receiving portion.
Conversely, if the refractive power of the lens is made greater to increase the quantity of light to the light-receiving portion, the angle of incidence onto the array will become greater in the lengthwise direction of the array 9. This will not only result in loss of the quantity of light, but also will vary the length of the optical path, which in turn will result in a blurred image. This tendency becomes greater toward the end in the lengthwise direction.
Similarly, in the case of slit exposure, the slit light beam is of a substantially rectangular shape and therefore, it is not preferable that the refractive power in the lengthwise direction of the slit light beam become too great. This tendency becomes more remarkable as the ratio of the length of the slit light beam in the lengthwise direction thereof and the slit width of the slit light beam becomes greater.